We would never cut the floorpan,but would rather weld a good stiff rail 2X2 or 2X3 minimually 1/8 thick from the trans crossmember area to heavy frame section at the rear.We add a 3"X6" flange to the rail at the front and weld it to the crossmember for eliminating any point load or twist that can tear the stock metal.You can further weld the driveshaft loop and rear suspension(ladder bar and etc) cross member to these rails to further stiffen the chassis,gussets can also be added at critical areas.If your going to start wacking and welding on the floor pans we suggest unloading the sprung weight and jigging the chassis on a frame machine.